In recent years, applications of liquid crystal display devices to mobile devices, typically to mobile phones and notebook computers, are rapidly advancing. Among the liquid crystal display devices, reflective-type liquid crystal display devices configured to reflect light incoming from the outside, such as ambient light, for display are capable of low power consumption and thin light-weight body because of no need for a backlight as a light source and are therefore especially suitable to mobile applications. However, on the other hand, the reflection-type liquid crystal display devices disadvantageously provide darker display because of the absence of a backlight.
To increase the display brightness of the reflection-type liquid crystal display devices, it is necessary to increase the reflection intensity in a direction perpendicular to a display screen which is a major viewing direction while making the reflection intensity have an appropriate angle distribution. For example, use of a specular reflection layer increases the regular reflection intensity but results in a problem of glare such that a background, such as a face of a viewer, is reflected in the display. That is, the problem is that the background is seen to be superimposed on the displayed images. Therefore, the reflection characteristics required for the reflection-type liquid crystal display devices do not include specular reflection but appropriate diffuse reflection. Appropriately-diffused light enables paper white display.
In view of such circumstances, attempts have been made up to now to improve the reflection characteristics of the reflection-type liquid crystal display devices. The reflection-type liquid crystal display devices widely used as of now is of such a type which includes a single polarizing plate on the viewer's side and which uses light reflected by a reflection layer provided on the rear face side of the liquid crystal layer (opposite to the viewer's side) for display. The reflection-type liquid crystal display devices referred to in this specification are also of this type unless stated otherwise.
For example, Patent Document 1 discloses a reflection-type liquid crystal display device in which the surface of pixel electrodes formed of a high-reflectance metal, such as aluminum, has an uneven shape such that the pixel electrodes have diffuse reflection characteristics. The reflection-type liquid crystal display devices of this type have a disadvantage of cost increase due to additional steps for the formation of appropriate unevenness in the surface of the pixel electrodes. They have other disadvantages, such as nonuniform orientation of liquid crystal molecules, varying retardation, etc., because the surface which is in contact with the liquid crystal layer has an unevenness.
Meanwhile, a reflection-type liquid crystal display device is under development in which a specular reflection layer formed of a high-reflectance metal such as aluminum (e.g., specular reflection pixel electrode) is used, and an extra element is added for providing diffuse reflection characteristics.
For example, Patent Document 2 discloses a reflection-type liquid crystal display device in which a forward scattering film is disposed on the outer side (viewer's side) of a viewer-side polarizing plate. The forward scattering film includes a light scattering layer made of a polymer binder containing spherical particles dispersed therein, the spherical particles having the average particle diameter of 1 μm to 10 μm. The relative refractive index of the spherical particles to the binder, n, is set to a value exceeding 0.91 and smaller than 1.09. Such a scattering layer which utilizes the difference in refractive index is sometimes referred to as “internal scattering layer”. Patent Document 2 says that, with the view of achieving excellent paper whiteness, the haze ratio of the forward scattering film is preferably 30% or higher and that, with the view of displaying images without obscuration, the image clarity measured by a transmission method is preferably 60% or higher (paragraphs [0037] to [0039]).
Patent Document 3 discloses a reflection-type liquid crystal display device in which a surface of the viewer-side polarizing plate on the outer side is used as a light scattering surface. The light scattering surface is formed by making a light transmitting resin surface uneven. The average height of the unevenness is 1 to 5 μm, the average pitch is 5 to 40 μm, and the haze ratio is 9 to 14% (FIG. 2, paragraph [0045]). When the haze ratio of the light scattering surface is 25% or higher, the problem of unclear displayed images cannot be prevented. When the haze ratio is 6% or lower, the problem of reflection of external images (background) in displayed images cannot be prevented (paragraph [0074]). Herein, a surface having an unevenness which exhibits light scatterability as the light scattering surface described in Patent Document 3 does is sometimes referred tows “scattering surface”.
Patent Document 4 discloses a reflection-type liquid crystal display device which includes a light scattering layer on the outer side of the viewer-side polarizing plate, the light scattering layer including an internal scattering layer and a scattering surface. Patent Document 4 says that the depth of unevenness of the light scattering layer is preferably 0.05 μm to 10 μm. Patent Document 4 describes, as an example, a light scattering layer which is formed of an acrylic resin (refractive index: 1.5) containing granular MgF2 of 0.2 μm (refractive index: 1.38) dispersed therein and which has an unevenness with the depth of 0.1 μm to 0.5 μm (paragraph [0035]). Patent Document 4 says that use of such a light scattering layer prevents reflection of background in display (phenomenon where a virtual image of the light source is observed) and specular reflection (regular reflection) at the outermost surface (paragraphs [0009] to [0010], [0013] to [0015], and [0022]).
[Patent Document 1] Japanese Laid-Open Patent Publication No. H5-323371
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2000-199809
[Patent Document 3] Japanese Laid-Open Patent Publication No. H7-306408
[Patent Document 4] Japanese Laid-Open Patent Publication No. H7-104272 (Japanese Patent No. 2898860)
[Patent Document 5] Japanese National Phase PCT Laid-Open Publication No. 2001-517319
[Patent Document 6] WO2006/059686A1